I have touched my finger to the input pin of transistors or logic gates, and then saw 60 cycle pulses at the output. My body picks up mains hum (from everywhere in the room), and carries it to the input.
When I remove my finger the pulses usually stop. However they may continue if a length of wire is attached to the input pin. The wire (or my body) is like an antenna picking up mains hum.
This isn't exactly capacitive sensing, and I doubt that touchpads operate on this principle. Nevertheless it's easy to experiment with.
A pullup/down resistor may be needed at the input to make operation reliable. Say 1 or 10 Mohm.